1. Field of the Invention
The present invention relates to a spindle state detector of an air bearing machine tool which is installed on a machine tool of which spindle is supported by an air bearing to detect a bearing gap state between the spindle and the housing which is a bearing portion of the air bearing, and a contact avoidance controller of the air bearing machine tool for avoiding a contact between the spindle and the housing. The present invention can be applied for conducting relatively high-load cutting process using a machine tool of which spindle is supported by a static-pressure air bearing.
2. Description of Related Art
Conventionally, a machine tool is used for cutting a metal material and the like. The machine tool cuts the metal material and the like by rotating a cutting tool together with the spindle, such as an end mill and a milling cutter attached to a spindle rotatably supported.
As the machine tool, a static-air bearing machine tool of which spindle is supported by a housing by a static-air bearing is known. According to the static-air bearing machine tool, since the spindle is supported by a compressed air layer provided between the housing as a bearing portion and the spindle, a friction between the spindle and the housing is diminished, thereby rotating the spindle at a high-speed to speed up cutting process.
[Spindle Displacement Problem of the Static-air bearing Machine Tool]
Sometimes, a great load is applied to the spindle of the static-air bearing machine tool during the cutting process. In this case, since the spindle is supported by the compressed air layer having elasticity, the spindle can be shifted from an original rotation axis by the load. When a displacement of the rotation axis gets larger, the spindle and the housing come into contact causing damage such as a seizure. Such a damage can be a fatal trouble of a machine tool.
Accordingly, the static-air bearing machine tool must be used under a condition where the spindle and the housing do not contact with each other. However, the condition has to be set so as to be safe to operate the static-air bearing machine tool, yet considering the fluctuation range of it, since the load applied to the spindle always fluctuates. In this case, full ability of the machine tool can not be utilized.
In solving the above problem, an alarm system and a contact avoidance controller are strongly desired, where a relative displacement of the spindle and the housing, i.e., a gap between the spindle and the housing during the rotation of the spindle is always monitored to transmit a danger signal when possible contact is predicted, or where the contact is positively avoided.
[Detection of the Spindle State]
A sensor which can precisely detect the relative displacement between the spindle and the housing without making contact with each other is necessary for continuously surveying the gap between the spindle and the housing.
As a sensor for detecting the relative displacement between the spindle and the housing, without making contact with each other an eddy current type, electro-capacitance type and laser-beam type sensor are known.
However, all of such conventional sensors can detect a displacement only in a single axial direction. Accordingly, multiple sensors have to be provided to monitor the movement of the spindle when the spindle as a target of the monitor is moved in three dimensions.
In other words, according to the conventional sensors when a spindle 11 is moved three-dimensionally relatively to a housing 13 by an external force F as shown in FIG. 17, a total of five sensors composed of a set of sensors Sx, Sy and Sz for respectively measuring a relative position of the housing 13 and the spindle 11 in the X, Y and Z-axis direction and another set of sensors Sa and Sb for measuring an inclination of the spindle 11 relative to the housing 13 together with the sensors Sx and Sy, have to be used to monitor the three-dimensional movement of the spindle 11.
Accordingly, multiple sensors have to be disposed in the static-air bearing machine tool to construct a system for monitoring the movement of the spindle to avoid contact. Therefore, the system is complicated.
Furthermore, since the system is complicated, it is difficult to newly introduce the system to a static-air bearing machine tool having no surveillance system.
[Contact Avoidance of the Spindle]
On the other hand, a contact avoidance controller having a detecting means for detecting the relative displacement of the spindle and the housing by a sensor and a controlling means for stopping a feed of the spindle when the detected relative displacement of the spindle and the housing is below a predetermined level is conventionally known as contact avoidance controller for avoiding the contact between the spindle and the housing in advance.
When the spindle and the housing are about to contact, the contact avoidance controller stops feeding the spindle or a table onto which a work is put. Accordingly, the load applied to the spindle is lessened to return the spindle to an original rotation axis.
The eddy current type, electro-capacitance type and laser-beam type sensors are used as sensors for the contact avoidance controller.
However, the following disadvantages are caused by the contact avoidance controller of the static-pressure air-bearing machine tool:
(1) The conventional contact avoidance controller only can stop the feed of the spindle or the table. Accordingly, the processing of the work is stopped during the control by the controller, thereby deteriorating processing efficiency of the machine tool; and
(2) Since the sensor of the conventional contact avoidance controller can only detect a displacement in one axis direction, two sensors for measuring the inclination of the spindle as well as sensors for measuring X, Y and Z-axis displacement are necessary when the measured spindle is moved in three dimensions, thereby complicating a structure of the detecting means.